The invention relates to a radio transmitter, a radio receiver, combinations thereof, and methods suitable for operating the apparatuses, in particular for synchronizing and/or ranging by means of UWB (Ultra Wide Band) signals.
In modern radio location systems and radio identification systems, ultra-wideband (UWB) signals are increasingly being used. The term UWB is used according to the definition of the US Federal Communications Commission (FCC) in cases where signal bandwidth is either at least 20% of the center frequency of the signal, or it is wider than 500 MHz.
A problem with UWB systems is the generation and detection of the UWB signals. In generating UWB signals, stringent legal requirements must be adhered to, and the signal spectra must be within strictly defined frequency masks. In the publications of the FCC or the European Electronic Communication Committee (ECC), for example, such requirements on the spectral masks are published. In common UWB systems, very short pulses (pulse duration typically in the range of between 100 ps-1 ns) are used as signals, and comparatively low pulse repetition rates (1-100 MHz) are used. The chosen mark-to-space ratios of typically 1:100 are necessary so that the signals generated have the very low average power to comply with legal requirements.
Due to the very short pulse durations and compounded by the long pulse separation it is difficult, however, to synchronize the signals of two UWB radio stations. This synchronization is usually carried out by means of special hardware correlators. These hardware correlators are necessary, because due to the extreme bandwidth of the UWB signals, it has hitherto not been possible to cheaply digitize the signals with an analog-to-digital converter to carry out the correlation, or the synchronization, by means of software on a purely computational basis. One of the drawbacks of signal comparison with hardware correlators is that the correlation for various offset points can only be sequentially determined and therefore on the one hand requires time—i.e., the synchronization can only be carried out in a step-wise or slow manner—and on the other hand also unnecessary amounts of power are consumed since a great number of signals need to be transmitted for the synchronization process to sequentially find the synchronization optimum—i.e., the correlation maximum.
A software correlation would be much more advantageous since only one UWB signal would need to be transmitted and received to compute a complete correlation and to find the correlation maximum. It is not possible, however, to implement this approach in a low-cost manner since, with large signal bandwidths, the necessary hardware preconditions are lacking, or are extremely expensive.
As has already been explained, current UWB systems often work with pulse signals and very simple modulation types, such as pulse position modulation or amplitude modulation. Basic principles are disclosed, for example, in “Terence W. Barrett “History of UltraWideBand (UWB) Radar & Communications: Pioneers and Innovators; http://www.ntia.doc.gov/osmhome/uwbtestplan/barret_history_(piersw-figs).pdf”. One of the first publications in which especially UWB location systems were treated, is U.S. Pat. No. 5,748,891. Further descriptions of UWB location systems can be found in U.S. Pat. Nos. 6,054,950; 6,300,903; and 6,483,461.
Simple pulse systems mean that it is exceedingly complicated to selectively shape the spectra of the pulses generated. Usually, and in particular with the planned European Admission Regulations, it is necessary that the pulses have a very clearly defined envelope, such as a Gaussian or cos2-shaped envelope, so that they remain within the spectral masks required by the regulation authorities and generate an extremely small amount of power in the side bands. Such selective amplitude control within such short pulse times is very difficult to implement technically, however.
For the reasons mentioned, newer UWB systems increasingly use more complex modulation types as an alternative, such as OFDM modulation. Since herein the baseband signals are mostly generated by a D/A converter, it has been necessary to limit the signals to a relatively small bandwidth, or to distribute the signals to various subbands, since D/A converters today do not efficiently allow direct generation of signals, for example, with a bandwidth of several GHz. An approach already discussed is, for example, the socalled UWB-MB-OFDM, disclosed, for example, in “Ultra-wideband communications: an idea whose time has come” Liuqing Yang; Giannakis, G. B., Signal Processing Magazine, IEEE Volume 21, Issue 6, November 2004 Page(s): 26-54”. Herein, the available spectrum is subdivided in a plurality of bands, and the information is transmitted within each band by means of OFDM modulation.
From German Patent Document No. DE 101 57 931 C2, a possibility for synchronizing radio stations for FMCW systems is known, wherein continuous waves are transmitted and received. A switch serves as a duplexer i.e. as a switch between transmitting and receiving operation. The duplexer is not for signal generation, however, but only for switching between transmitting and receiving.
U.S. Pat. No. 2,379,395 A also shows a switch which forms a duplexer filter as a duplexer. A method is described for frequency stabilizing in a data/communication system with analog frequency modulation, i.e. classical radio technology. The method is only for frequency stabilization of a pure communication system, wherein no synchronization of clocks of different system components is mentioned.
From International Patent Publication No. WO 2005/098465 A2, a method for synchronizing clock means on the basis of FMCW systems is known, wherein continuous waves are transmitted and received.
From German Patent Document No. DE 199 46 161 A1, a method for ranging is known on the basis of FMCW systems, wherein continuous waves are transmitted and received.